In conventional dry cleaning machines, articles soiled with dirt, stains and other contaminants are put into a cylindrical basket mounted for rotation inside the cleaning chamber through its door, after which the door is closed to seal the chamber. The sealed chamber is then partially filled with a hydrocarbon cleaning fluid or "solvent". The articles and fluid are tumbled together by rotating the basket or "tumbler" so that the solvent is brought into intimate contact with the surfaces of the article and is absorbed by the absorbent portions thereof to dissolve or suspend contaminants so that they are removed from the articles. The solvent may be continuously filtered and recirculated to the cleaning chamber to remove suspended but undissolved solid contaminants. After the cleaning cycle, the solvent is drained from the cleaning chamber and may be filtered and reused, provided that the level of dissolved contaminants is not approaching saturation.
It is now recognized that such conventional dry cleaning machines and methods involve significant health and environmental risks because of the toxic nature of conventional hydrocarbon cleaning fluids. Because carbon dioxide is an environmentally safe gas present in ambient air, its efficient use as a dry cleaning fluid would avoid many of the environmental and health problems associated with conventional cleaning fluids.
Efforts have been made in the past to develop dry cleaning systems in which clothing is contacted with chilled liquid carbon dioxide for the purpose of removing contaminants from the garments. Such prior art systems contemplate converting the carbon dioxide from its liquid state to its gaseous state in an evaporator so as to leave behind the contaminants. The gas is then condensed back to liquid carbon dioxide, which is recycled to the cleaning chamber.
However, keeping carbon dioxide in its liquid state requires that the cleaning chamber be at a relatively high pressure in a pressure vessel. This requirement has made it difficult to agitate the clothing sufficiently to achieve efficient and cost effective cleaning operations. For example, prior art devices have been unable to use the rotating tumbler of conventional dry cleaning machines because of difficulties in preventing high pressure carbon dioxide from escaping around the drive shaft connected to the tumbler through a wall of the pressure vessel. Suggested alternatives, such as rotating a basket or other agitator through a magnetic coupling, have been found to be relatively ineffective, especially for commercial size units.
Accordingly, a real need exists for a liquid carbon dioxide dry cleaning system that is efficient and capable of providing effective agitation of the articles to be cleaned and the pressurized cleaning medium without significant loss of the medium from the pressure vessel. The efficiency of such a system also requires minimizing the amount of carbon dioxide expended during the cleaning operations by maximizing the amount recovered and reused after each washing and rinsing cycle.